1. Technical Field of the Invention
This invention relates generally to data communications and more particularly to synchronization and scrambling of frames of Ethernet data communications.
2. Description of Related Art
As is known, Ethernet is a standard that defines data conveyance protocols for data transmissions in Local Area Networks (LAN) and has multiple variations thereof depending on desired data rates and transmission media. The Ethernet standard IEEE 802.3 defines a variety of physical layer specifications for metallic and fiber-optic cables. For example, the specifications for metallic twisted-pair cables include 10 BASE-T (10 megabit-per-second (Mbit/s)), 100 BASE-TX (100 Mbit/s), and 1000 BASE-T (1 gigabit-per-second (Gbit/s)). A new specification 10GBASE-T (10 Gbit/s) is under development.
LANs based on the Ethernet standard typically exhibit a star topology with point-to-point links between a switch or hub and attached stations, although in some cases, a bus topology may be employed.
As is also known, transmitted modulation symbols must be randomized to avoid data-dependent spectral characteristics of the transmitted signal and allow for proper timing recovery and synchronization. The process of randomization is also known as scrambling. There exist two forms of scrambling, self-synchronizing scrambling and cipher-stream scrambling. With self-synchronizing scrambling, a sequence of binary symbols is divided by a scrambling polynomial in the transmitter and then multiplied by the same scrambling polynomial in the receiver. The advantage of this scrambling method is that no additional synchronization is required. On the other hand, self-synchronization has the disadvantage of error multiplication because every erroneously received binary symbol induces a finite number of additional errors in the descrambled binary sequence. With cipher-stream scrambling, in the transmitter a pseudo-random scrambling sequence is added modulo-2 to the sequence of binary data symbols. In the receiver, the same scrambling sequence is again added modulo-2 to recover the original binary data sequence. In the case of transmission errors no error multiplication occurs, which makes cipher-stream scrambling a preferred scrambling method. However, cipher-stream scrambling requires the synchronous generation of the same scrambling sequence in the transmitter and the receiver.
Therefore, a need exists for a method and apparatus which enables the efficient generation of a scrambling sequence for cipher-stream scrambling and descrambling in frame based data transmission systems.